Liquids can be filtered to remove undissolved particulate matter. In addition, dissolved solids can be removed from a liquid by sorption. The term "sorption" includes adsorption, whereby dissolved solids become associated with a surface of a material, called a sorbent; or absorption, whereby dissolved solids become localized within the body of the sorbent. For example, water may be filtered to remove particulates, such as sand, and dissolved minerals may be removed by sorption so that the water is suitable for drinking. In addition, the filtration and/or sorption of liquids plays a significant role in wastewater treatment and pharmaceutical production, among others. As used herein, the word "particulates" shall mean solid chemical compounds that remain undissolved in the liquid to be purified.
One way that particulates can be removed from a liquid is by filtration; that is, the liquid is passed through a filter and the opening or pores that are part of the filter's structure allow the liquid to pass through the filter, but do not allow the particulates to pass through. The size and the amount of particulates filtered from a liquid can be optimized by varying the openings or pore sizes of the filter.
On the other hand, dissolved solids usually pass through the openings or pores of a filter even when the openings or pores are very small. In polar liquids, such as water, many substances that dissolve end up as ions in solution. For example, an amount of sodium chloride (NaCl) dissolves in water at room temperature to form sodium ions, which are cations because they have a positive charge, and chlorine ions, which are anions because they have a negative charge. One way to remove such ions from solution is through the use of an ion exchange material.
Ion exchange is the reversible interchange of ions between a solid material and a liquid in which there is no permanent change in the structure of the solid material. Conventional ion exchange resins contain ion-active sites throughout their structure. A cation exchange resin has a negatively charged structure and exchanges cations. Conversely, an anion exchange resin has a positively charged structure and exchanges anions. When a liquid containing ions is passed through an ion exchange resin, the ions in the liquid are exchanged with the ions contained in the structure of the ion exchange resin. Thus, the ions that were in the liquid become ionically bonded to the ion exchange resin, and the ions that were originally present in the resin are liberated from the resin and become part of the liquid. Through ion exchange, certain ions can be removed from a liquid.
Before the present invention, filter elements were made by winding layers of fibers and/or yarn around a foraminous center core. The core was typically perforated to allow the liquid to flow in and out of the core. The fibers and/or yarn that was wrapped around the core to form winding layers did not possess ion exchange capability, but instead, were used to create openings or pores of various sizes for filtering suspended particulate matter. In addition, the winding layers also provided a substrate upon which finely divided particulate ion exchange resins could be deposited, and the winding layer the farthest from the core, the outermost layer, was usually precoated with the particulate ion exchange resin. Then, the liquid to be purified was passed through the ion exchange resin and the winding layers. Examples of such filter elements are disclosed in U.S. Pat. Nos. 4,414,113 and 4,269,707, which are assigned to the same assignee as the present invention.
To purify large volumes of liquids, the above-described filter element was typically part of a filter unit in which more than one filter element was used. Several drawbacks, however, are associated with the above-described ion exchange resin precoated filter elements. First, the presence of the particulate ion exchange resin precoated on the winding layers resulted in slower flow rates of the liquid through the filter elements. Also, if the flow is reversed through the filter to expel the particulate matter that has been filtered from the liquid to be purified, the particulate ion exchange resin is also expelled from the winding layers making it necessary to replace the ion exchange resin if the filter is to be used again. Moreover, the ion exchange precoat must be discarded after use, as it is not practical to regenerate. As a result, a large volume of waste is generated that must be discarded, which can be expensive.
The present invention, which relates to a filter element having a foraminous center core around which is wound at least one layer of yarn that has ion exchange capability, circumvents some of these problems. For example, it may not be necessary to apply a separate ion exchange resin if the winding layer itself has ion exchange capability. Therefore, there may not be a need to reapply the ion exchange resin upon reversal of flow to dislodge filtered particulates. Moreover, the step of precoating the filter element with particulate ion exchange resin may no longer be necessary. The present invention, therefore, provides for a quick and efficient way to make a filter element having both the ability to filter out particulates and having ion exchange capabilities. A preferred liquid to be purified is water.
In addition, filters having ion exchange capacity have been used as catalysts for chemical reactions. However, the kinetics of particulate ion exchange resins are limited. Greatly improved kinetics are, however, provided by the use of ion exchange fibers, which can be pan of an ion exchange yarn.
Another problem associated with the use of particulate ion exchange resins precoated on the winding layers of a filter element is channeling. As filtering of a liquid proceeds, the distribution of ion exchange particles in a precoat can shift causing channels to form in the precoat. Typically, the channels may contain little or no ion exchange resin. Thus, the liquid which passes through the channels is not effectively ion exchanged as desired.